1. Field of the Invention
The present invention relates to alkylation or alkenylation of the side chain of substituted aromatic compounds. In particular, the present invention comprises a process for alkylating the side-chain of xylene with a C4-alkylene or diene.
The present invention also relates to a novel catalyst comprising a restructured clay.
2. Description of Related Art
The alkylation (or alkenylation) of side-chain(s) of alkyl-substituted aromatic hydrocarbons by aliphatic olefins is carried out in the presence of a basic catalyst. An acidic substance, on the other hand, catalyses the alkylation of the benzene ring. Typically, the alkylation reaction is carried out batch-wise or in a fixed-bed reactor in at moderate to high temperatures. In the batch reactor the catalyst is usually present as a slurry.
In prior art, the basic catalysts have typically comprised an alkali metal, optionally on a carrier. Sodium and potassium are the most widely used alkali metals (e.g. H. Pines, J.A.C.S., 1955, 77, 5), and the most typical carriers have been magnesia and alumina, although early examples of the catalysts used for side-chain alkylation include sodium-loaded potassium phosphate (U.S. Pat. No. 5,347,062). U.S. Pat. No. 5,118,895 discloses a catalyst comprising potassium on magnesia and U.S. Pat. No. 5,097,088 discloses a catalyst comprising potassium on magnesia-alumina.
The obvious problem with using metallic alkali metals is their reactivity. It is generally known that alkali metals react vigorously with even small amounts of water. Thus, the use of metallic metal catalysts in an industrial scale process sets very strict requirements to the handling of the starting materials and process operation.
Alumina is useful as a carrier due to its high surface area enabling a good dispersability of the loaded metal. Nevertheless, because of the acidic nature of the alumina, a catalyst comprising conventionally produced alumina as a carrier cannot provide sufficient activity for alkylating the side-chain of aromatic compounds.
On the contrary, a high activity is achieved when a catalyst comprising an alkali metal loaded on a basic carrier is used. For example, alkali-metal impregnated on supports such as Na-exchanged zeolite have been suggested in prior art, but the yields of alkylated product remained extremely low. Thus, the activity of basic catalysts does not result in high yields of reaction products. It can thus be concluded that the basic carriers used to date have been of low surface area and the alkali metal loaded on them is not sufficiently dispersed.
Further, when the catalyst is active enough, problems of a different kind may occur. For example, if the alkylbenzene which is alkylated has more than one side-chain, it is probable that both these side-chains are alkylated—via continued reaction of the alkylating agent present with initial reaction product formed. In some cases the desired product must be removed from the catalyst.
An attempt has previously (U.S. Pat. No. 4,990,717) been made to solve the problem by using both stirred tank and catalytic distillation with fixed catalyst bed, and separating mono-alkenylated product generated from unreacted alkylbenzene and/or C4 to C5 conjugated diene present, both up-stream or downstream. Unreacted alkylbenzene and conjugated diene obtained could then be recycled to the catalyst bed for further alkenylation.
Smectite clays in pillared form have previously been disclosed in Italian Patent Application No. RM 98A 000130. The publication discloses the preparation of catalysts comprising pillared smectite clays to which metal ions have been exchanged. These catalysts are used for catalysing dehydrogenation and in the alkylation reaction of benzene rings.